The improvement of the strength properties of PTFE in the form of sheet, tape or rod by processes involving heating and stretching is well known. Particularly important has been the development of the process of "expansion" of PTFE by W. L. Gore & Associates, Inc. (Gore), which involves stretching an unsintered PTFE article at a high rate of stretch at an elevated temperature, to cause expansion (i.e. density reduction, with increase of porosity) of the PTFE. The isothermal expansion stage is followed by increase of temperature, with the material retained in its expanded state, to at least 327.degree. C. which effects the phenomenon known as amorphous locking or node-locking, by the formation of amorphous regions in the PTFE which appear to lock together the fibrils of PTFE. This process is described for example in Gore's U.S. Pat. Nos. 3,953,566 and 3,962,153, where examples are given of biaxial and uniaxial stretching of films, and uniaxial stretching of rods. Sheets having two-dimensional strength, i.e. substantially equal tensile strengths in two perpendicular directions, are subjected only to biaxial stretching.
Non-contact heating of an extruded PTFE article while it is being stretched is also disclosed in GB-A-2025835.
A different process, involving sintering of the PTFE before stretching is described in EP-A-391887. GB-A-1525980 describes a process of peeling a curtain of threads from a sintered PTFE block, and stretching the thread curtain while it passes over a curved heated surface. The threads are then brought together to form a cable or yarn. The individual threads are very fine.
It is well known that unsintered PTEE articles, such as extruded tape, differ much in their behaviour in heat treatment from thermoplastic polyolefins such as polyethylene. For completeness of prior art disclosure in relation to the invention to be described below, there is also mentioned GB-A-1287874 which describes a process of producing fibres from a polymer film in which the film is fed to a combined slitting and stretching zone in which apparently the cutting device is heated. The cut fibres are stretched by taking them up from the cutting tool at a rate faster than the film feed rate. After stretching, the fibres pass over a curved guide surface, which may be heated. Although PTFE is mentioned, the only detailed example employs polypropylene film.
In GB-A-1541681, an orientable polymer strip is passed over a heated fixed metal tube, to provide a limited heating region in a stretching zone. PTFE is mentioned, but the examples disclose use of polyethylene, and the aim of the technique is to minimize the width reduction occurring on stretching while achieving high orientation of the polymer.
GB-A-1124109 discloses passing a polyolefin body over a straight pressure edge, which is heated, and stretching it so that a necking shoulder forms at the pressure edge. Biaxially oriented ribbons and tapes are obtained. Polyethylene and polypropylene are employed in the examples.
The application of expanded PTFE material as dental floss is described in U.S. Pat. No. 4,776,358, in which material obtainable from Gore in the form of tape is used to form an elongate envelope containing cleaning material. The exterior surface of this PTFE envelope is not coated.
A second dental floss made of PTFE is described in EP-A-335466. Expanded PTFE of a particular strength produced by the Gore expansion process is coated with microcrystalline wax.
Another dental floss based on PTFE not made by the Gore expansion process is described in WO 92/10978 (Westone).
The use of PTFE in dental floss is also mentioned briefly in U.S. Pat. No. 4,836,226.
It is desirable to employ, in a dental floss, a PTFE material which in use is resistant to fibrillation, i.e. the separation of fibrils from the main body of the material.